1. Field of the Invention
This invention relates to a method of producing a biaxially oriented film of thermoplastic resin and an apparatus therefor, wherein a tubular thermoplastic resin film is simultaneously stretched in both the longitudinal and transversal directions of the film, and more particularly, is utilized for stabilizing in stretching operation.
2. Description of the Prior Art
As the methods of producing the biaxially oriented film of thermoplastic resin, there have heretofore been well known two methods including a tenter method, and a tubular method, wherein a tubular film is held between two pairs of top and bottom nip rolls, which are different in circumferential speed from each other, the tubular film is heated between these nip rolls to a temperature where the tubular film can be stretched, and gas is delivered into the tubular film, whereby, due to pressure of the gas delivered and the difference in circumferential speed between the two pairs of nip rolls, the tubular film is simultaneously stretched in both the longitudinal and transversal directions.
As for the former tenter method, there have been known a multistage biaxially stretching method and a simultaneously biaxially stretching method. When a polyamide film to be biaxially stretched is produced according to the multistage biaxially stretching method for example, firstly, hydrogen bonding is generated which has a directional property in a direction perpendicular to a direction, in which the film has been stretched initially, and the succeeding orienting in a direction perpendicular to the initial orienting direction tends to become a neck stretching, so that it is very difficult to obtain a uniformly oriented film. Furthermore, when a polyamide film to be biaxially stretched is produced according to the simultaneously biaxially stretching method, a side edge portion of the raw material film is grasped by a grip, thereby presenting such disadvantages that this portion does not contribute to the stretching, which leads to a loss to a considerable extent, whereby the yield is lowered to a great extent, and the manufacturing cost due to the large-sized stretching mechanical equipment, including an increased installation floor area.
According to the tubular method, the longitudinal and transversal stretchings occur simultaneously, and setting and changing of stretching ratios can be easily carried out, so that the tubular method has such great features that a well-balanced film can be obtained and the installation cost is low. On the other hand, in this method, it is most important to uniformly heat the tubular film in a heating-stretching section to stabilize an stretching starting point. However, it is very difficult to secure this. From the above reason, despite the advantage of low installation cost, etc. of the tubular method, in fact, the bulk of the biaxially oriented film manufacturing method continues to belong to the tenter method.
Now, in order to obviate the disadvantage of the tubular method, there has been proposed a method, wherein positions before and behind a stretching zone are shielded with covers, shaking of the tube is avoided by an upward air stream, to thereby preclude the adverse influence of the open air (Japanese Patent Application Publication No. 6834/1964). However, even with this method, particularly, at about a portion of a bubble neck, i.e. about the stretching starting point, the flowrate of air flowing along the film is lowered, and heat transfer performed between the film of this portion and the open air lacks uniformity in the circumferential direction of the tube, so that the temperature distribution in the circumferential direction of the tubular film is extended around the stretching starting point in particular, thus resulting in insufficient stability and uniformity of the stretching.
Therefore, there have been proposed methods, wherein there are provided two air rings which blow air against about this stretching starting point in directions opposite to each other (for example, Japanese Patent Application Publication Nos. 27480/1970, 15439/1971, 47269/1974 and the like). However, in these methods, because two air rings are needed, portions of air blown out of the air rings impinge on each other to cause turbulences in the air streams, air heated beyond a predetermined temperature (for example, 150.degree. C.) is needed, such disadvantages present that an equipment for heating is complicated, heating efficiency is lowered due to a contact conductive heating of air and heating energy is lost by the fact that air heated is incontinent.
Furthermore, there has been proposed a method, wherein a raw material film is previously heated to 50.degree.-90.degree. C., thereafter, atmospheric temperature between an stretching starting point and a stretching ending point is held at 180.degree.-250.degree. C. and the stretching is carried out with a difference in stretching ratio between the longitudinal and transversal directions being maintained at 0.2-0.6 (Japanese Patent Application Publication No. 15914/78). However, according to this method, it is necessary that the raw material film is previously heated and the atmospheric temperature posterior to the stretching starting point is heated as high as to 180.degree.-250.degree. C., and no finesse is reflected in the uniform heating at the orienting starting point and the stabilization of the tubular film before and after the orienting, and hence, these problems have not been solved yet.
Now, out of the thermoplastic resin films, a film made of a saponified material of ethylene-vinyl acetate copolymer (hereinafter referred to as "EVOH") is excellent in gas barrier properties, resistance to oils, antistatic properties and the like, and useful as a packaging material for food products and pharmaceuticals and the like. However, in the biaxial stretching of this EVOH film hydrogen bonding is strong, and the structure tends to be fixed, so that a strong stretching force is needed, and broken film, neck stretching and the like tend to occur. As the result, if the tubular method is used as it is, then an industrially useful and excellent oriented film cannot be obtained.
From this reason, various special methods have been proposed. For example, there have been proposed a method, wherein stretching is performed with a film being closely attached to another film capable of being stretched (Japanese Patent Kokai (Laid-Open) No. 6276/1976), another method, wherein a moisture containing value of an unoriented film is increased, whereby stretching is performed under the specified stretching conditions (Japanese Patent Kokai (Laid-Open) Nos. 88067/1978, 129776/1977 and Patent Application Publication No. 43199/1978) and so forth.
However, in the former method, it is indispensable to use another film. In the latter method, it takes much time to increase the moisture containing value of the EVOH film to 4% or more, and moreover, it is very difficult to control the moisture containing value, so that the latter method cannot be called an established industrially manufacturing method.
From the above reason, there has been proposed a method, wherein an unoriented film having a moisture containing value of 2% or less, which has been obtained by a normal manufacturing of the EVOH film, is previously heated at 50.degree.-70.degree. C. for 5 sec or less, thereafter, rapidly heated to 70.degree.-100.degree. C. by use of heated air, and biaxially stretched (Japanese Patent Kokai (Laid-Open) No. 25920/1982). However, according to this method, previous heating is needed, the film is rapidly heated within a narrow temperature range of 70.degree.-100.degree. C., and moreover, this rapid heating is performed by air-blast, thus presenting such disadvantages that the thermal efficiency is low and the temperature control is difficult to perform.